Modulation of a light beam can be used to create an optical signal that represents digital or analog information. A common technique for such modulation is external modulation, which uses a light source such as a stabilized laser to generate a constant light beam and an optical modulator to alter or modulate a property of the light beam. Typically, the amplitude, phase, or polarization of the beam is modulated to represent the information being conveyed in the optical signal. Light from the light source or the optical modulator can be coupled into a waveguide for guided signal transmission before or after modulation.
A variety of devices and techniques are known for both modulation of an optical signal and coupling of an optical signal into a waveguide. For example, two common types of modulators are acousto-optic modulators and electro-optic modulators. An acousto-optic modulator (AOM), which is sometimes called a Bragg cell, uses the acousto-optic effect where sound waves cause variations in the refractive index of a material, and the variations diffract light passing through the material. The intensity of light at a specific angle of diffraction thus varies, i.e., is modulated according to changes in the acoustic signal. Electro-optic modulators similarly use the electro-optic effect to modulate the intensity of light at a target angle of diffraction. Optical couplers typically employ optical elements to focus light beam on to an end of a waveguide and are often needed for the transition of an optical signal between a device such as an optical modulator and a waveguide through which the optical signal is transmitted (i.e., to couple light in or out).
Use of the same reference symbols in different figures indicates similar or identical items.